Stable DNA transfection of the primitive protozoan pathogen Giardia lamblia

Nourseothricin as a novel drug for selection of transgenic Giardia lamblia

Functional gene and protein characterizations in parasitic protists are often limited by their genetic tractability. Despite the development of CRISPR-Cas9-derived or inspired approaches for a handful of protist parasites, the overall genetic tractability of these organisms remains limited. The intestinal parasite Giardia lamblia is one such species, with the added challenge of a paucity of reliable selection markers. To address this limitation, we tested the feasibility of using Nourseothricin as an effective selection agent in Giardia. Here, we report that axenically-grown WB Giardia cells are sensitive to Nourseothricin and that engineering expression of the streptothricin acetyltransferase (SAT-1) gene from Streptomyces rochei in transgenic parasites confers resistance to this antibiotic. Furthermore, we determine that SAT-1-expressing parasites are cross-resistant neither to Neomycin nor Puromycin, which are widely used to select for transgenic parasites. Consequently, we show that Nourseothricin can be used in sequential combination with both Neomycin and Puromycin to select for dual transfection events. This work increases the number of reliable selection agents and markers for Giardia genetic manipulation, expanding the limited molecular toolbox for this species of global medical importance.

A myeloid leukemia factor homolog is involved in tolerance to stresses and stress-induced protein metabolism in Giardia lamblia

BACKGROUND

The eukaryotic membrane vesicles contain specific sets of proteins that determine vesicle function and shuttle with specific destination. Giardia lamblia contains unknown cytosolic vesicles that are related to the identification of a homolog of human myeloid leukemia factor (MLF) named MLF vesicles (MLFVs). Previous studies suggest that MLF also colocalized with two autophagy machineries, FYVE and ATG8-like protein, and that MLFVs are stress-induced compartments for substrates of the proteasome or autophagy in response to rapamycin, MG132, and chloroquine treatment. A mutant protein of cyclin-dependent kinase 2, CDK2m3, was used to understand whether the aberrant proteins are targeted to degradative compratments. Interestingly, MLF was upregulated by CDK2m3 and they both colocalized within the same vesicles. Autophagy is a self-digestion process that is activated to remove damaged proteins for preventing cell death in response to various stresses. Because of the absence of some autophagy machineries, the mechanism of autophagy is unclear in G. lamblia.

RESULTS

In this study, we tested the six autophagosome and stress inducers in mammalian cells, including MG132, rapamycin, chloroquine, nocodazole, DTT, and G418, and found that their treatment increased reactive oxygen species production and vesicle number and level of MLF, FYVE, and ATG8-like protein in G. lamblia. Five stress inducers also increased the CDK2m3 protein levels and vesicles. Using stress inducers and knockdown system for MLF, we identified that stress induction of CDK2m3 was positively regulated by MLF. An autophagosome-reducing agent, 3-methyl adenine, can reduce MLF and CDK2m3 vesicles and proteins. In addition, knockdown of MLF with CRISPR/Cas9 system reduced cell survival upon treatment with stress inducers. Our newly developed complementation system for CRISPR/Cas9 indicated that complementation of MLF restored cell survival in response to stress inducers. Furthermore, human MLF2, like Giardia MLF, can increase cyst wall protein expression and cyst formation in G. lamblia, and it can colocalize with MLFVs and interact with MLF.

CONCLUSIONS

Our results suggest that MLF family proteins are functionally conserved in evolution. Our results also suggest an important role of MLF in survival in stress conditions and that MLFVs share similar stress-induced characteristics with autophagy compartments.

Efficient CRISPR/Cas9-mediated gene disruption in the tetraploid protist Giardia intestinalis

CRISPR/Cas9-mediated genome editing has become an extremely powerful technique used to modify gene expression in many organisms, including parasitic protists. Giardia intestinalis, a protist parasite that infects approximately 280 million people around the world each year, has been eluding the use of CRISPR/Cas9 to generate knockout cell lines due to its tetraploid genome. In this work, we show the ability of the in vitro assembled CRISPR/Cas9 components to successfully edit the genome of G. intestinalis. The cell line that stably expresses Cas9 in both nuclei of G. intestinalis showed effective recombination of the cassette containing the transcription units for the gRNA and the resistance marker. This highly efficient process led to the removal of all gene copies at once for three independent experimental genes, mem, cwp1 and mlf1. The method was also applicable to incomplete disruption of the essential gene, as evidenced by significantly reduced expression of tom40. Finally, testing the efficiency of Cas9-induced recombination revealed that homologous arms as short as 150 bp can be sufficient to establish a complete knockout cell line in G. intestinalis.

Giardia duodenalis: Biology and Pathogenesis

Giardia duodenalis captured the attention of Leeuwenhoek in 1681 while he was examining his own diarrheal stool, but, ironically, it did not really gain attention as a human pathogen until the 1960s, when outbreaks were reported. Key technological advances, including in vitro cultivation, genomic and proteomic databases, and advances in microscopic and molecular approaches, have led to an understanding that this is a eukaryotic organism with a reduced genome rather than a truly premitochondriate eukaryote. This has included the discovery of mitosomes (vestiges of mitochondria), a transport system with many of the features of the Golgi apparatus, and even evidence for a sexual or parasexual cycle. Cell biology approaches have led to a better understanding of how Giardia survives with two nuclei and how it goes through its life cycle as a noninvasive organism in the hostile environment of the lumen of the host intestine. Studies of its immunology and pathogenesis have moved past the general understanding of the importance of the antibody response in controlling infection to determining the key role of the Th17 response. This work has led to understanding of the requirement for a balanced host immune response that avoids the extremes of an excessive response with collateral damage or one that is unable to clear the organism. This understanding is especially important in view of the remarkable ranges of early manifestations, which range from asymptomatic to persistent diarrhea and weight loss, and longer-term sequelae that include growth stunting in children who had no obvious symptoms and a high frequency of postinfectious irritable bowel syndrome (IBS).

Transfection With Plasmid Causing Stable Expression of a Foreign Gene Affects General Proteome Pattern in Giardia lamblia Trophozoites

Giardia lamblia is an important causative agent of persistent diarrhea in humans, domestic animals, and cattle. Basic research is usually performed with the strain WBC6 and includes genetic manipulations such as transfections. Here, we investigate how transfection with a plasmid causing stable expression of a foreign gene affects the whole proteome pattern. Using shotgun mass spectrometry, we compare the proteomes of untransfected trophozoites to trophozoites transfected with Escherichia coli glucuronidase A (GusA). Besides GusA, which is detected in the transfected trophozoites only, the proteomes of untransfected and transfected trophozoites differ by 132 differentially expressed proteins. In particular, transfection induces antigenic variation. Since transfection causing stable expression affects the proteome pattern, transfection experiments should take into account this effect. Due to a unique peptide panel, GusA is an example for a suitable internal standard for experiments involving transfected cells. Data are available via ProteomeXchange with identifier PXD022565.

Comparative proteomics of three Giardia lamblia strains: investigation of antigenic variation in the post-genomic era

Giardia lamblia is a causative agent of persistent diarrhoea widespread in regions with low hygienic standards. Laboratory research is based on cloned lines issuing from various patient isolates typed in the late 1980s and 90s using restriction analysis and serology. In the present study, we compared the well-characterized strain WBC6 with another clone of the parent WB isolate termed WBA1 and with a clone from another isolate, GS/M-83-H7, using shotgun mass spectrometry proteomics. We identified 398 proteins differentially expressed between the GS and both WB isolates and 97 proteins differentially expressed between the two WB isolates. We investigated the expression levels of the predominant variant-specific surface proteins (VSPs) in each clone and matched the previously described major VSPs of each strain to the corresponding open reading frame sequences identified by whole-genome sequencing efforts. Furthermore, since the original WB isolate comes from a patient treated with metronidazole, we compared the susceptibilities of the strains to nitro compounds, as well the expression levels of enzymes involved in nitro reduction and on the corresponding enzyme activities and found distinct differences between the three strains.

DNA topoisomerase IIIβ promotes cyst generation by inducing cyst wall protein gene expression in Giardia lamblia

Giardia lamblia causes waterborne diarrhoea by transmission of infective cysts. Three cyst wall proteins are highly expressed in a concerted manner during encystation of trophozoites into cysts. However, their gene regulatory mechanism is still largely unknown. DNA topoisomerases control topological homeostasis of genomic DNA during replication, transcription and chromosome segregation. They are involved in a variety of cellular processes including cell cycle, cell proliferation and differentiation, so they may be valuable drug targets. Giardia lamblia possesses a type IA DNA topoisomerase (TOP3β) with similarity to the mammalian topoisomerase IIIβ. We found that TOP3β was upregulated during encystation and it possessed DNA-binding and cleavage activity. TOP3β can bind to the cwp promoters in vivo using norfloxacin-mediated topoisomerase immunoprecipitation assays. We also found TOP3β can interact with MYB2, a transcription factor involved in the coordinate expression of cwp1-3 genes during encystation. Interestingly, overexpression of TOP3β increased expression of cwp1-3 and myb2 genes and cyst formation. Microarray analysis confirmed upregulation of cwp1-3 and myb2 genes by TOP3β. Mutation of the catalytically important Tyr residue, deletion of C-terminal zinc ribbon domain or further deletion of partial catalytic core domain reduced the levels of cleavage activity, cwp1-3 and myb2 gene expression, and cyst formation. Interestingly, some of these mutant proteins were mis-localized to cytoplasm. Using a CRISPR/Cas9 system for targeted disruption of top3β gene, we found a significant decrease in cwp1-3 and myb2 gene expression and cyst number. Our results suggest that TOP3β may be functionally conserved, and involved in inducing Giardia cyst formation.

Recent advances in functional research in Giardia intestinalis

This review considers current advances in tools to investigate the functional biology of Giardia, it's coding and non-coding genes, features and cellular and molecular biology. We consider major gaps in current knowledge of the parasite and discuss the present state-of-the-art in its in vivo and in vitro cultivation. Advances in in silico tools, including for the modelling non-coding RNAs and genomic elements, as well as detailed exploration of coding genes through inferred homology to model organisms, have provided significant, primary level insight. Improved methods to model the three-dimensional structure of proteins offer new insights into their function, and binding interactions with ligands, other proteins or precursor drugs, and offer substantial opportunities to prioritise proteins for further study and experimentation. These approaches can be supplemented by the growing and highly accessible arsenal of systems-based methods now being applied to Giardia, led by genomic, transcriptomic and proteomic methods, but rapidly incorporating advanced tools for detection of real-time transcription, evaluation of chromatin states and direct measurement of macromolecular complexes. Methods to directly interrogate and perturb gene function have made major leaps in recent years, with CRISPr-interference now available. These approaches, coupled with protein over-expression, fluorescent labelling and in vitro and in vivo imaging, are set to revolutionize the field and herald an exciting time during which the field may finally realise Giardia's long proposed potential as a model parasite and eukaryote.

Development of CRISPR/Cas9-mediated gene disruption systems in Giardia lamblia

Giardia lamblia becomes dormant by differentiation into a water-resistant cyst that can infect a new host. Synthesis of three cyst wall proteins (CWPs) is the fundamental feature of this differentiation. Myeloid leukemia factor (MLF) proteins are involved in cell differentiation, and tumorigenesis in mammals, but little is known about its role in protozoan parasites. We developed a CRISPR/Cas9 system to understand the role of MLF in Giardia. Due to the tetraploid genome in two nuclei of Giardia, it could be hard to disrupt a gene completely in Giardia. We only generated knockdown but not knockout mutants. We found that knockdown of the mlf gene resulted in a significant decrease of cwp gene expression and cyst formation, suggesting a positive role of MLF in encystation. We further used mlf as a model gene to improve the system. The addition of an inhibitor for NHEJ, Scr7, or combining all cassettes for gRNA and Cas9 expression into one plasmid resulted in improved gene disruption efficiencies and a significant decrease in cwp gene expression. Our results provide insights into a positive role of MLF in inducing Giardia differentiation and a useful tool for studies in Giardia.

Roles of end-binding 1 protein and gamma-tubulin small complex in cytokinesis and flagella formation of Giardia lamblia

Giardia lamblia is a unicellular organism with two nuclei, a median body, eight flagella, and an adhesive disk. γ‐Tubulin is a microtubule (MT)‐nucleating protein that functions in the γ‐tubulin small complex (γ‐TuSC) in budding yeast. In this study, G. lamblia γ‐tubulin (Glγ‐tubulin) was found to bind to another MT‐binding protein, namely G. lamblia end‐binding protein 1 (GlEB1), via both in vivo and in vitro assays. Hemagglutinin (HA)‐tagged Glγ‐tubulin localized to the basal bodies, axonemes, and median bodies of G. lamblia trophozoites. The knockdown of Glγ‐tubulin expression using an anti‐Glγ‐tubulin morpholino resulted in a decreased growth rate and an increased failed cytokinesis cells of Giardia. The formation of median bodies was affected, and the central pair of MTs in flagella was frequently missing in the Giardia treated with an anti‐Glγ‐tubulin morpholino. G. lamblia γ‐tubulin complex protein 2 (GlGCP2) and GlGCP3, which are putative components of γ‐TuSC, were co‐immunoprecipitated with HA‐tagged Glγ‐tubulin in Giardia extracts. The knockdown of GlGCP2 and GlGCP3 expression also resulted in decreased formation of both the median body and flagella MTs. Knockdown of Glγ‐tubulin, GlGCP2, and GlGCP3 expression affected localization of GlEB1 in G. lamblia. In addition, decreased level of GlEB1 caused reduced formation of median body and the central pair of flagella MTs. These results indicated that Glγ‐tubulin plays a role in MT nucleation for median body formation and flagella biogenesis as a component of Glγ‐TuSC in Giardia and GlEB1 may be involved in this process.

Cyst-Wall-Protein-1 is fundamental for Golgi-like organelle neogenesis and cyst-wall biosynthesis in Giardia lamblia

The genome of the protozoan parasite Giardia lamblia is organized in two diploid nuclei, which has so far precluded complete analysis of gene function. Here we use a previously developed Cre/loxP-based knock-out and selection marker salvage strategy in the human-derived isolate WB-C6 to eliminate all four copies of the Cyst-Wall-Protein-1 locus (CWP1). Because these loci are silenced in proliferating trophozoites and highly expressed only in encysting cells, CWP1 ablation allows functional characterization of a conditional phenotype in parasites induced to encyst. We show that encysting Δcwp1 cells are unable to establish the stage-regulated trafficking machinery with Golgi-like encystation-specific vesicles required for cyst-wall formation but show morphological hallmarks of cyst development and karyokinesis. This ‘pseudocyst' phenotype is rescued by transfection of Δcwp1 cells with an episomally maintained CWP1 expression vector. Genome editing in genera Giardia and Trypanosoma are the only reported examples addressing questions on pathogen transmission within the Excavata supergroup.

Giardia lamblia is a human protozoan parasite with two diploid nuclei, which makes complete knock-out of a gene of interest challenging. Here the authors use a Cre/loxP-based approach to knock-out cyst-wall protein 1 (cwp1) and show that CWP1 is essential for cyst-wall biosynthesis.

Localizing Proteins in Fixed Giardia lamblia and Live Cultured Mammalian Cells by Confocal Fluorescence Microscopy

Confocal fluorescence microscopy and electron microscopy (EM) are complementary methods for studying the intracellular localization of proteins. Confocal fluorescence microscopy provides a rapid and technically simple method to identify the organelle in which a protein localizes but only EM can identify the suborganellular compartment in which that protein is present. Confocal fluorescence microscopy, however, can provide information not obtainable by EM but required to understand the dynamics and interactions of specific proteins. In addition, confocal fluorescence microscopy of cells transfected with a construct encoding a protein of interest fused to a fluorescent protein tag allows live cell studies of the subcellular localization of that protein and the monitoring in real time of its trafficking. Immunostaining methods for confocal fluorescence microscopy are also faster and less involved than those for EM allowing rapid optimization of the antibody dilution needed and a determination of whether protein antigenicity is maintained under fixation conditions used for EM immunogold labeling. This chapter details a method to determine by confocal fluorescence microscopy the intracellular localization of a protein by transfecting the organism of interest, in this case Giardia lamblia, with the cDNA encoding the protein of interest and then processing these organisms for double label immunofluorescence staining after chemical fixation. Also presented is a method to identify the organelle targeting information in the presequence of a precursor protein, in this case the presequence of the precursor to the Euglena light harvesting chlorophyll a/b binding protein of photosystem II precursor (pLHCPII), using live cell imaging of mammalian COS7 cells transiently transfected with a plasmid encoding a pLHCPII presequence fluorescent protein fusion and stained with organelle-specific fluorescent dyes.

The Cre/loxP system in Giardia lamblia: genetic manipulations in a binucleate tetraploid protozoan

The bacteriophage-derived Cre/loxP system is a valuable tool that has revolutionised genetic and cell biological research in many organisms. We implemented this system in the intestinal parasite Giardia lamblia, an evolutionarily diverged protozoan whose binucleate and tetraploid genome organisation severely limits the application of reverse genetic approaches. We show that Cre-recombinase is functionally expressed in G. lamblia and demonstrate "recycling" of selectable markers. Providing the means for more complex and versatile genetic modifications, this technique massively increases the scope of functional investigations in G. lamblia and other protozoa with similar limitations with respect to genetic manipulation.

Transient reporter gene expression in oocysts and sporozoites of Cryptosporidium parvum controlled by endogenous promoters

The apicomplexan protozoan Cryptosporidium parvum is an enteric parasite that affects a variety of mammal hosts including humans, and causes serious diarrheal disease in immunocompromised individuals, notably AIDS patients. Despite many advances in the development of transgenic techniques in many protozoan parasites over the past two decades, rare reports have been documented on the genetic manipulation on C. parvum. Achievement of the DNA-based transfection chiefly depends on the selection of an effective parasite genus-specific promoter. This report described the successful yellow (YFP-YFP) or red (RFP) fluorescent protein expression in oocysts and sporozoites of C. parvum controlled by the endogenous promoters of actin, alpha tubulin, and myosin genes using the restricted enzyme-mediated integration technique. One expression cassette in pBluescript backbone, YFP-YFP or RFP fused between 5' and 3' untranslated regions of actin gene, displayed the highest transfection efficiency with fluorescence rate around 50%. The established DNA-based transient transfection assay may contribute to a better understanding of the biology of Cryptosporidium species and their relationship with hosts and may also result in the development of more efficient molecule-based vaccines and drugs.

Giardia intestinalis incorporates heme into cytosolic cytochrome b₅

The anaerobic intestinal pathogen Giardia intestinalis does not possess enzymes for heme synthesis, and it also lacks the typical set of hemoproteins that are involved in mitochondrial respiration and cellular oxygen stress management. Nevertheless, G. intestinalis may require heme for the function of particular hemoproteins, such as cytochrome b5 (cytb5). We have analyzed the sequences of eukaryotic cytb5 proteins and identified three distinct cytb5 groups: group I, which consists of C-tail membrane-anchored cytb5 proteins; group II, which includes soluble cytb5 proteins; and group III, which comprises the fungal cytb5 proteins. The majority of eukaryotes possess both group I and II cytb5 proteins, whereas three Giardia paralogs belong to group II. We have identified a fourth Giardia cytb5 paralog (gCYTb5-IV) that is rather divergent and possesses an unusual 134-residue N-terminal extension. Recombinant Giardia cytb5 proteins, including gCYTb5-IV, were expressed in Escherichia coli and exhibited characteristic UV-visible spectra that corresponded to heme-loaded cytb5 proteins. The expression of the recombinant gCYTb5-IV in G. intestinalis resulted in the increased import of extracellular heme and its incorporation into the protein, whereas this effect was not observed when gCYTb5-IV containing a mutated heme-binding site was expressed. The electrons for Giardia cytb5 proteins may be provided by the NADPH-dependent Tah18-like oxidoreductase GiOR-1. Therefore, GiOR-1 and cytb5 may constitute a novel redox system in G. intestinalis. To our knowledge, G. intestinalis is the first anaerobic eukaryote in which the presence of heme has been directly demonstrated.

DNA topoisomerase II is involved in regulation of cyst wall protein genes and differentiation in Giardia lamblia

The protozoan Giardia lamblia differentiates into infectious cysts within the human intestinal tract for disease transmission. Expression of the cyst wall protein (cwp) genes increases with similar kinetics during encystation. However, little is known how their gene regulation shares common mechanisms. DNA topoisomerases maintain normal topology of genomic DNA. They are necessary for cell proliferation and tissue development as they are involved in transcription, DNA replication, and chromosome condensation. A putative topoisomerase II (topo II) gene has been identified in the G. lamblia genome. We asked whether Topo II could regulate Giardia encystation. We found that Topo II was present in cell nuclei and its gene was up-regulated during encystation. Topo II has typical ATPase and DNA cleavage activity of type II topoisomerases. Mutation analysis revealed that the catalytic important Tyr residue and cleavage domain are important for Topo II function. We used etoposide-mediated topoisomerase immunoprecipitation assays to confirm the binding of Topo II to the cwp promoters in vivo. Interestingly, Topo II overexpression increased the levels of cwp gene expression and cyst formation. Microarray analysis identified up-regulation of cwp and specific vsp genes by Topo II. We also found that the type II topoisomerase inhibitor etoposide has growth inhibition effect on Giardia. Addition of etoposide significantly decreased the levels of cwp gene expression and cyst formation. Our results suggest that Topo II has been functionally conserved during evolution and that Topo II plays important roles in induction of the cwp genes, which is key to Giardia differentiation into cysts.

Giardia lamblia becomes infective by differentiation into water-resistant cysts. During encystation, cyst wall proteins (CWPs) are highly synthesized and are targeted to the cyst wall. However, little is known about the regulation mechanisms of these genes. DNA topoisomerases can resolve the topological problems and are needed for a variety of key cellular functions, including cell proliferation, cell differentiation and organ development in higher eukaryotes. We found that giardial Topo II was highly expressed during encystation. Topo II is present in Giardia nuclei and is associated with the encystation-induced cwp gene promoters. Topo II has typical DNA cleavage activity of type II topoisomerases. Interestingly, overexpression of Topo II can induce cwp gene expression and cyst formation. Addition of a type II topoisomerase inhibitor, etoposide, significantly decreased the levels of cwp gene expression and cyst formation. Etoposide also has growth inhibition effect on Giardia. Our results suggest that Topo II plays an important role in induction of encystation by up-regulation of the cwp gene expression. Our results provide insights into the function of Topo II in parasite differentiation into cysts and help develop ways to interrupt the parasite life cycle.

Stable transfection and continuous expression of heterologous genes in Entamoeba invadens

Amoebiasis is spread by the ingestion of dormant Entamoeba histolytica cysts. Intervention of encystation could break the transmission cycle, thereby reducing disease burden. The model system used to study trophozoite to cyst differentiation is Entamoeba invadens. Here we describe an electroporation-based method for stable transfection of E. invadens with a plasmid pEiNEO-LUC containing the neomycin phosphotransferase gene under the control of E. invadens ribosomal protein gene S10 promoter. The plasmid also contains luciferase reporter gene expressed from the promoter of ribosomal protein gene L3. After electroporation, cells receiving the plasmid were selected by growth in 10μgml(-1) G418 and stable lines were obtained in four to five weeks. The plasmid was replicated episomally to ∼10 copies per haploid genome. In the absence of drug selection 50% of the plasmid copies were lost in 9-10 days. In cells growing under drug selection the reporter gene was continuously expressed throughout the differentiation process from trophozoite to cyst and back, making this system suitable for analysis of genes involved in differentiation.

Functional redundancy of two Pax-like proteins in transcriptional activation of cyst wall protein genes in Giardia lamblia

The protozoan Giardia lamblia differentiates from a pathogenic trophozoite into an infectious cyst to survive outside of the host. During encystation, genes encoding cyst wall proteins (CWPs) are coordinately induced. Pax family transcription factors are involved in a variety of developmental processes in animals. Nine Pax proteins have been found to play an important role in tissue and organ development in humans. To understand the progression from primitive to more complex eukaryotic cells, we tried to identify putative pax genes in the G. lamblia genome and found two genes, pax1 and pax2, with limited similarity. We found that Pax1 may transactivate the encystation-induced cwp genes and interact with AT-rich initiatior elements that are essential for promoter activity and transcription start site selection. In this study, we further characterized Pax2 and found that, like Pax1, Pax2 was present in Giardia nuclei and it may specifically bind to the AT-rich initiator elements of the encystation-induced cwp1-3 and myb2 genes. Interestingly, overexpression of Pax2 increased the cwp1-3 and myb2 gene expression and cyst formation. Deletion of the C-terminal paired domain or mutation of the basic amino acids of the paired domain resulted in a decrease of nuclear localization, DNA-binding activity, and transactivation activity of Pax2. These results are similar to those found in the previous Pax1 study. In addition, the profiles of gene expression in the Pax2 and Pax1 overexpressing cells significantly overlap in the same direction and ERK1 associated complexes may phosphorylate Pax2 and Pax1, suggesting that Pax2 and Pax1 may be downstream components of a MAPK/ERK1 signaling pathway. Our results reveal functional redundancy between Pax2 and Pax1 in up-regulation of the key encystation-induced genes. These results illustrate functional redundancy of a gene family can occur in order to increase maintenance of important gene function in the protozoan organism G. lamblia.

The minimal proteome in the reduced mitochondrion of the parasitic protist Giardia intestinalis

The mitosomes of Giardia intestinalis are thought to be mitochondria highly-reduced in response to the oxygen-poor niche. We performed a quantitative proteomic assessment of Giardia mitosomes to increase understanding of the function and evolutionary origin of these enigmatic organelles. Mitosome-enriched fractions were obtained from cell homogenate using Optiprep gradient centrifugation. To distinguish mitosomal proteins from contamination, we used a quantitative shot-gun strategy based on isobaric tagging of peptides with iTRAQ and tandem mass spectrometry. Altogether, 638 proteins were identified in mitosome-enriched fractions. Of these, 139 proteins had iTRAQ ratio similar to that of the six known mitosomal markers. Proteins were selected for expression in Giardia to verify their cellular localizations and the mitosomal localization of 20 proteins was confirmed. These proteins include nine components of the FeS cluster assembly machinery, a novel diflavo-protein with NADPH reductase activity, a novel VAMP-associated protein, and a key component of the outer membrane protein translocase. None of the novel mitosomal proteins was predicted by previous genome analyses. The small proteome of the Giardia mitosome reflects the reduction in mitochondrial metabolism, which is limited to the FeS cluster assembly pathway, and a simplicity in the protein import pathway required for organelle biogenesis.

Rapid tagging and integration of genes in Giardia intestinalis

We developed a series of plasmids that allow C-terminal tagging of any gene in its endogenous locus in Giardia intestinalis, with different epitope tags (triple hemagglutinin [3HA] and triple Myc [3Myc]) and selection markers (puromycin, neomycin, and a newly developed marker, blasticidin). Using these vectors, cyclin B and aurora kinase were tagged, expressed, and localized.

A novel pax-like protein involved in transcriptional activation of cyst wall protein genes in Giardia lamblia

Giardia lamblia differentiates into infectious cysts to survive outside of the host. It is of interest to identify factors involved in up-regulation of cyst wall proteins (CWPs) during this differentiation. Pax proteins are important regulators of development and cell differentiation in Drosophila and vertebrates. No member of this gene family has been reported to date in yeast, plants, or protozoan parasites. We have identified a pax-like gene (pax1) encoding a putative paired domain in the G. lamblia genome. Epitope-tagged Pax1 localized to nuclei during both vegetative growth and encystation. Recombinant Pax1 specifically bound to the AT-rich initiator elements of the encystation-induced cwp1 to -3 and myb2 genes. Interestingly, overexpression of Pax1 increased cwp1 to -3 and myb2 gene expression and cyst formation. Deletion of the C-terminal paired domain or mutation of the basic amino acids of the paired domain resulted in a decrease of the transactivation function of Pax1. Our results indicate that the Pax family has been conserved during evolution, and Pax1 could up-regulate the key encystation-induced genes to regulate differentiation of the protozoan eukaryote, G. lamblia.

A novel family of cyst proteins with epidermal growth factor repeats in Giardia lamblia

Background

Giardia lamblia parasitizes the human small intestine to cause diarrhea and malabsorption. It undergoes differentiation from a pathogenic trophozoite form into a resistant walled cyst form. Few cyst proteins have been identified to date, including three cyst wall proteins (CWPs) and one High Cysteine Non-variant Cyst protein (HCNCp). They are highly expressed during encystation and are mainly targeted to the cyst wall.

Methodology and Principal Findings

To identify new cyst wall proteins, we searched the G. lamblia genome data base with the sequence of the Cryptosporidium parvum oocyst wall protein as a query and found an Epidermal Growth Factor (EGF)-like Cyst Protein (EGFCP1). Sequence analysis revealed that the EGF-like repeats of the EGFCP1 are similar to those of the tenascin family of extracellular matrix glycoproteins. EGFCP1 and HCNCp have a higher percentage of cysteine than CWPs, but EGFCP1 has no C-terminal transmembrane region found in HCNCp. Like CWPs and HCNCp, the EGFCP1 protein (but not transcript) was expressed at higher levels during encystation and it was localized to encystation-specific vesicles in encysting trophozoites. Like HCNCp, EGFCP1 was localized to the encystation-specific vesicles, cyst wall and cell body of cysts, suggesting that they may share a common trafficking pathway. Interestingly, overexpression of EGFCP1 induced cyst formation and deletion of the signal peptide from EGFCP1 reduced its protein levels and cyst formation, suggesting that EGFCP1 may help mediate cyst wall synthesis. We also found that five other putative EGFCPs have similar expression profiles and similar locations and that the cyst formation was induced upon their overexpression.

Conclusions and Significance

Our results suggest that EGFCPs may function like cyst wall proteins, involved in differentiation of G. lamblia trophozoites into cysts. The results lead to greater understanding of parasite cyst walls and provide valuable information that helps develop ways to interrupt the G. lamblia life cycle.

The biological goal of Giardia lamblia life cycle is differentiation into a cyst form (encystation) that can survive in the environment and infect a new host. Since cystic stages are key to transmission of parasites, this differentiation may be a target for interruption of the life cycle. Synthesis and assembly of the extracellular cyst wall are the major hallmarks of this important differentiation. During encystation, cyst wall structural proteins are coordinately synthesized and are mainly targeted to the cyst wall. However, only a few such proteins have been identified to date. In this study, we used a combination of bioinformatics and molecular approaches to identify new cyst structural proteins from G. lamblia and found a group of Epidermal Growth Factor (EGF)-like Repeats containing Cyst Proteins (EGFCPs). Interestingly, the levels of EGFCPs proteins increased significantly during encystation, which matches the characteristics of the Giardia cyst wall protein. Further characterization and localization studies suggest that EGFCPs may function like cyst wall proteins, involved in differentiation of G. lamblia trophozoites into cysts. Our results provide valuable information regarding the function of a new group of cyst proteins in parasite differentiation into cysts and help develop ways to interrupt the parasite life cycle.

The monothiol single-domain glutaredoxin is conserved in the highly reduced mitochondria of Giardia intestinalis

The highly reduced mitochondria (mitosomes) of Giardia intestinalis are recently discovered organelles for which, it was suggested, iron-sulfur cluster assembly was their only conserved function. However, only an incomplete set of the components required for FeS cluster biogenesis was localized to the mitosomes. Via proteomic analysis of a mitosome-rich cellular fraction together with immunofluorescence microscopy, we identified a novel mitosomal protein homologous to monothiol glutaredoxins containing a CGFS motif at the active site. Sequence analysis revealed the presence of long nonconserved N-terminal extension of 77 amino acids, which was absent in the mature protein. Expression of the complete and N-terminally truncated forms of the glutaredoxin indicated that the extension is involved in glutaredoxin import into mitosomes. However, the mechanism of preprotein processing is unclear, as the mitosomal processing peptidase is unable to cleave this type of extension. The recombinant mature protein was shown to form a homodimeric structure, which binds a labile FeS cluster. The cluster is stabilized by glutathione and dithiothreitol. Phylogenetic analysis showed that giardial glutaredoxin is related to the mitochondrial monothiol glutaredoxins involved in FeS cluster assembly. The identification of a mitochondrial-type monothiol glutaredoxin in the mitosomes of G. intestinalis thus completes the mitosomal FeS cluster biosynthetic pathway and provides further evidence for the mitochondrial origin of these organelles.

UPF1, a conserved nonsense-mediated mRNA decay factor, regulates cyst wall protein transcripts in Giardia lamblia

The Giardia lamblia cyst wall is required for survival outside the host and infection. Three cyst wall protein (cwp) genes identified to date are highly up-regulated during encystation. However, little is known of the molecular mechanisms governing their gene regulation. Messenger RNAs containing premature stop codons are rapidly degraded by a nonsense-mediated mRNA decay (NMD) system to avoid production of non-functional proteins. In addition to RNA surveillance, NMD also regulates thousands of naturally occurring transcripts through a variety of mechanisms. It is interesting to know the NMD pathway in the primitive eukaryotes. Previously, we have found that the giardial homologue of a conserved NMD factor, UPF1, may be functionally conserved and involved in NMD and in preventing nonsense suppression. In this study, we tested the hypothesis that NMD factors can regulate some naturally occurring transcripts in G. lamblia. We found that overexpression of UPF1 resulted in a significant decrease of the levels of CWP1 and cyst formation and of the endogenous cwp1-3, and myb2 mRNA levels and stability. This indicates that NMD could contribute to the regulation of the cwp1-3 and myb2 transcripts, which are key to G. lamblia differentiation into cyst. Interestingly, we also found that UPF1 may be involved in regulation of eight other endogenous genes, including up-regulation of the translation elongation factor gene, whose product increases translation which is required for NMD. Our results indicate that NMD factor could contribute to the regulation of not only nonsense containing mRNAs, but also mRNAs of the key encystation-induced genes and other endogenous genes in the early-diverging eukaryote, G. lamblia.

Regulation of cyst wall protein promoters by Myb2 in Giardia lamblia

Myb family transcription factors are important in regulating cell proliferation, differentiation, and cell cycle progression. Giardia lamblia differentiates into infectious cysts to survive outside of the host. During encystation, genes encoding cyst wall proteins (CWPs) are coordinately induced. We have identified an encystation-induced Myb2 protein, which binds to the promoter regions of the cwp genes and myb2 itself in vitro. To elucidate the role of Myb2 in G. lamblia, we tested the hypothesis that Myb2 can activate encystation-induced genes. We found that overexpression of Myb2 resulted in an increase of expression of CWP1 at both protein and mRNA levels. Interestingly, the Myb2-overexpressing trophozoites had increased capability to differentiate into cysts. In cotransfection assays, Myb2 was able to transactivate the cwp promoters and its own promoter in vivo, suggesting that its gene can be positively autoregulated. Moreover, deletion of the N- or C-terminal domain resulted in a decrease of transactivation and autoregulation function of Myb2. We also found that the promoter of a newly identified encystation-induced gene, the giardial myeloid leukemia factor-like gene, has the Myb2 binding sites and that its mRNA levels were increased by Myb2 overexpression. Chromatin immunoprecipitation assays confirmed that Myb2 was bound to the promoters with its binding sites. Transfection of the myb2 antisense construct reduced the levels of the cwp1 transcripts and cyst formation. Our results suggest that Myb2 is a potent transactivator of the cwp genes and other endogenous genes and plays an important role in G. lamblia differentiation into cysts.

Neomycin and puromycin affect gene expression in Giardia lamblia stable transfection

Two systems for stable transfection of Giardia have been established using selection either by neomycin or by puromycin. We asked if these selection systems themselves influenced expression of endogenous giardial genes. Northern blot analysis showed a approximately 1.4 to approximately 7-fold increase in the encystation-induced cyst wall protein 1 (cwp1), cwp2, and gmyb2 gene transcripts in the drug selected cell lines during vegetative growth, compared with untransfected cells. However, the levels of the constitutive ran, lrp3, or alpha2-tubulin gene transcripts decreased slightly or did not change in these stably transfected cell lines. Part of the effect could be due to drug selection, since treatment of untransfected cells with G418 or puromycin also had similar effects. Nuclear run-on assays showed that part of the effect comes from an increase in transcription initiation rate. The levels of CWP and cyst formation during vegetative growth also increased in the transfected cell lines. Using proteomic technologies, we identified eight genes whose expression is upregulated in neomycin selected cell lines, including phosphoglycerate kinase, glyceraldehyde-3-phosphate dehydrogenase, ornithine carbamoyltransferase, carbamate kinase, orf 16424, cyclophilin, co-chaperone-like p21, and bip. Six of these are also upregulated in puromycin selected cell lines. Our results indicate that transfection and drug selection, per se, can alter expression of genes involved in metabolism, protein folding, and differentiation status in Giardia.

A novel ARID/Bright-like protein involved in transcriptional activation of cyst wall protein 1 gene in Giardia lamblia

The capability of protozoan parasite Giardia lamblia to encyst is critical for survival outside the host and its transmission. AT-rich interaction domain (ARID) or Bright homologs constitute a large family of transcription factors in higher eukaryotes that regulate cell proliferation, development, and differentiation. We asked whether Giardia has ARID-like genes and whether they influence gene expression during Giardia encystation. Blast searches of the Giardia genome data base identified two genes with putative ARID/Bright domains (gARID1 and 2). Epitope-tagged gARID1 was found to localize to nuclei. Recombinant gARID1 specifically bound to the encystation-induced cyst wall protein (cwp) gene promoters. Mutation analysis revealed that AT-rich initiators were required for binding of gARID1 to the cwp promoters. gARID1 contains several key residues for DNA binding, and its binding sequences are similar to those of the known ARID family proteins. The gARID1 binding sequences were positive cis-acting elements of the cwp1 promoter during both vegetative growth and encystation. We also found that gARID1 transactivated the cwp1 promoter through its binding sequences in vivo. Our results suggest that the ARID family has been conserved during evolution and that gARID1 is an important transactivator in regulation of the Giardia cwp1 gene, which is key to Giardia differentiation into cysts.

Giardia mitosomes and trichomonad hydrogenosomes share a common mode of protein targeting

Mitochondria are archetypal organelles of endosymbiotic origin in eukaryotic cells. Some unicellular eukaryotes (protists) were considered to be primarily amitochondrial organisms that diverged from the eukaryotic lineage before the acquisition of the premitochondrial endosymbiont, but their amitochondrial status was recently challenged by the discovery of mitochondria-like double membrane-bound organelles called mitosomes. Here, we report that proteins targeted into mitosomes of Giardia intestinalis have targeting signals necessary and sufficient to be recognized by the mitosomal protein import machinery. Expression of these mitosomal proteins in Trichomonas vaginalis results in targeting to hydrogenosomes, a hydrogen-producing form of mitochondria. We identify, in Giardia and Trichomonas, proteins related to the component of the translocase in the inner membrane from mitochondria and the processing peptidase. A shared mode of protein targeting supports the hypothesis that mitosomes, hydrogenosomes, and mitochondria represent different forms of the same fundamental organelle having evolved under distinct selection pressures.

Influence of 5' sequences on expression of the Tet repressor in Giardia lamblia

Gene expression is poorly understood in Giardia lamblia. Previously we utilized the Escherichia coli tetracycline regulatory elements to develop a giardial-inducible gene expression system. In this study, we tested the hypothesis that regions flanking the tet repressor (tet R) may influence its expression and affect inducibility of the regulatory system. We found that addition of a 6-His tag or nuclear localization signal (NLS) at the N- but not C-terminus of tet R, increased the induction ratios >100-fold. A non-specific sequence also increased the induction ratio. Fusing NLS at the N-terminus, also led to exclusively nuclear tet R localization. Changing the promoter from gdh or alpha-giardin to alpha2-tubulin increased the induction ratio slightly. Tet R expression at both RNA and protein levels correlated with repression efficiency, indicating that coding sequences of the 6-His tag or NLS may contribute to transcriptional activation of the exotic tet R gene in Giardia. In addition, we found that the tet R system mediated gene repression and induction during encystation. Previous studies used an artificial reporter gene. In this study, we were able to induce overexpression of epitope-tagged cyst wall protein 1 (CWP1) in vegetatively growing Giardia trophozoites. Moreover, we could repress or induce expression of exogenous CWP1 in encysting cells. Taken together, our data suggest that expression of tet R in Giardia is complex and can be strongly influenced by additional sequences, especially at its N-terminus. This system provides insights into expression of an alien gene and can be exploited to regulate gene expression and study important functions in G. lamblia.

Giardia lamblia: stable expression of green fluorescent protein mediated by giardiavirus

Giardia lamblia, an early diverging eukaryote that infects several species including humans and a major agent of water-borne diarrhea throughout the world, can be infected with a double-stranded RNA virus, giardiavirus (GLV). A chimeric GLV cDNA and green fluorescent protein (GFP) according to the cis-acting signals of the GLV genome required for expression of foreign gene was constructed and its in vitro transcript was electroporated into GLV-infected G. lamblia trophozoites, GFP was expressed transiently. pGDH5/NEO/GLV was constructed by combining the neomycin resistance cassette in which the neomycin phosphotransferase gene was flanked by Giardia glutamate dehydrogenase (GDH) uncoding regions and the transcription cassette in which the chimera of GLV cDNA and GFP was located downstream from GDH gene promoter on a single plasmid. This plasmid was electroporated into G. lamblia and the transfectants persistently expressed GFP under G418 selection. This stable transfection system should provide a valuable tool for genetic study of G. lamblia.

An improved method for detection of Leishmania amastigotes by an antibody probe against the small subunit of leishmanial ribonucleotide reductase

By taking advantage of an antibody raised against the small M2 subunit of ribonucleotide reductase of Leishmania that reacts with the enzyme in the nucleus of the parasite but does not cross-react with the same enzyme of the host macrophage, an improved fluorescence-staining method is developed for enumeration of leishmanial amastigotes inside the macrophage. The method offers an accurate and easy way of counting, compared with Giemsa staining.

Mining the Giardia lamblia genome for new cyst wall proteins

The Giardia lamblia cyst wall (CW), which is required for survival outside the host and infection, is a primitive extracellular matrix. Because of the importance of the CW, we queried the Giardia Genome Project Database with the coding sequences of the only two known CW proteins, which are cysteine-rich and contain leucine-rich repeats (LRRs). We identified five new LRR-containing proteins, of which only one (CWP3) is up-regulated during encystation and incorporated into the cyst wall. Sequence comparison with CWP1 and -2 revealed conservation within the LRRs and the 44-amino-acid N-flanking region, although CWP3 is more divergent. Interestingly, all 14 cysteine residues of CWP3 are positionally conserved with CWP1 and -2. During encystation, C-terminal epitope-tagged CWP3 was transported to the wall of water-resistant cysts via the novel regulated secretory pathway in encystation-secretory vesicles (ESVs). Deletion analysis revealed that the four LRRs are each essential to target CWP3 to the ESVs and cyst wall. In a deletion of the most C-terminal region, fewer ESVs were stained in encysting cells, and there was no staining in cysts. In contrast, deletion of the 44 amino acids between the signal sequence and the LRRs or the region just C-terminal to the LRRs only decreased the number of cells with CWP3 targeting to ESVs and cyst wall by approximately 50%. Our studies indicate that virtually every portion of the CWP3 protein is needed for efficient targeting to the regulated secretory pathway and incorporation into the cyst wall. Further, these data demonstrate the power of genomics in combination with rigorous functional analyses to verify annotation.

The secretory apparatus of an ancient eukaryote: protein sorting to separate export pathways occurs before formation of transient Golgi-like compartments

Transmission of the protozoan parasite Giardia intestinalis to vertebrate hosts presupposes the encapsulation of trophozoites into an environmentally resistant and infectious cyst form. We have previously shown that cyst wall proteins were faithfully sorted to large encystation-specific vesicles (ESVs), despite the absence of a recognizable Golgi apparatus. Here, we demonstrate that sorting to a second constitutively active pathway transporting variant-specific surface proteins (VSPs) to the surface depended on the cytoplasmic VSP tail. Moreover, pulsed endoplasmic reticulum (ER) export of chimeric reporters containing functional signals for both pathways showed that protein sorting was done at or very soon after export from the ER. Correspondingly, we found that a limited number of novel transitional ER-like structures together with small transport intermediates were generated during encystation. Colocalization of transitional ER regions and early ESVs with coat protein (COP) II and of maturing ESVs with COPI and clathrin strongly suggested that ESVs form by fusion of ER-derived vesicles and subsequently undergo maturation by retrograde transport. Together, the data supported the hypothesis that in Giardia, a primordial secretory apparatus is in operation by which proteins are sorted in the early secretory pathway, and the developmentally induced ESVs carry out at least some Golgi functions.

Calcium signaling in excystation of the early diverging eukaryote, Giardia lamblia

Excystation of Giardia lamblia, which initiates infection, is a poorly understood but dramatic differentiation induced by physiological signals from the host. Our data implicate a central role for calcium homeostasis in excystation. Agents that alter cytosolic Ca(2+) levels (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-tetra(acetyloxymethyl) ester, a Ca(2+) channel blocker, Ca(2+) ionophores, and thapsigargin) strongly inhibit excystation. Treatment of Giardia with thapsigargin raised intracellular Ca(2+) levels, and peak Ca(2+) responses increased with each stage of excystation, consistent with the kinetics of inhibition. Fluorescent thapsigargin localized to a likely Ca(2+) storage compartment in cysts. The ability to sequester ions in membrane-bounded compartments is a hallmark of the eukaryotic cell. These studies support the existence of a giardial thapsigargin-sensitive Ca(2+) storage compartment resembling the sarcoplasmic/endoplasmic reticulum calcium ATPase pump-leak system and suggest that it is important in regulation of differentiation and appeared early in the evolution of eukaryotic cells. Calmodulin antagonists also blocked excystation. The divergent giardial calmodulin localized to the eight flagellar basal bodies/centrosomes, like protein kinase A. Inhibitor kinetics suggest that protein kinase A signaling triggers excystation, whereas calcium signaling is mainly required later, for parasite activation and emergence. Thus, the basal bodies may be a cellular control center to coordinate the resumption of motility and cytokinesis in excystation.

The cytoskeleton of Giardia lamblia

Giardia lamblia is a ubiquitous intestinal pathogen of mammals. Evolutionary studies have also defined it as a member of one of the earliest diverging eukaryotic lineages that we are able to cultivate and study in the laboratory. Despite early recognition of its striking structure resembling a half pear endowed with eight flagella and a unique ventral disk, a molecular understanding of the cytoskeleton of Giardia has been slow to emerge. Perhaps most importantly, although the association of Giardia with diarrhoeal disease has been known for several hundred years, little is known of the mechanism by which Giardia exacts such a toll on its host. What is clear, however, is that the flagella and disk are essential for parasite motility and attachment to host intestinal epithelial cells. Because peristaltic flow expels intestinal contents, attachment is necessary for parasites to remain in the small intestine and cause diarrhoea, underscoring the essential role of the cytoskeleton in virulence. This review presents current day knowledge of the cytoskeleton, focusing on its role in motility and attachment. As the advent of new molecular technologies in Giardia sets the stage for a renewed focus on the cytoskeleton and its role in Giardia virulence, we discuss future research directions in cytoskeletal function and regulation.

A novel Myb-related protein involved in transcriptional activation of encystation genes in Giardia lamblia

Giardia lamblia is an important human intestinal parasite that survives outside of the host by differentiation of trophozoites into infectious cysts. Transcriptional regulation is key for encystation gene expression, but the mechanisms are unknown. Giardia genome database searches identified a myb-like gene (gmyb2) whose expression increased during encystation. Epitope-tagged gMyb2 localized to both nuclei. DNA binding and mutation analysis showed that gMyb2 binds specifically to C(T/A)ACAG, a c-Myb-like target sequence in the promoters of encystation-induced genes encoding gMyb2, three cyst wall proteins and G6PI-B, a key enzyme in cyst wall polysaccharide biosynthesis. gMyb2 binding sites were not found in the upstream regions of 31 other giardial genes. Deletion of the putative gMyb2 binding site greatly reduced encystation-specific promoter activity of g6pi-b. Fusion of gMyb2 binding sites to the constitutive ran promoter or g6pi-b promoter deletion lacking the gMyb2 binding site in-duced encystation-specific expression. gMyb2 may play an important role in transcriptional regulation of encystation genes, and may help co-ordinate synthesis of cyst wall proteins and polysaccharide. gMyb2 is the first giardial transcription factor to be functionally identified and the first that is associated with upregulation of encystation genes. This work provides a model for study of differential gene regulation in early diverging eukaryotic organisms.

Surface antigenic variation in Giardia lamblia

Giardia lamblia, a common intestinal dwelling protozoan and a cause of diarrhoea in humans and animals world-wide, undergoes surface antigenic variation. The variant-specific surface proteins (VSPs) are a family of related, highly unusual proteins that cover the entire surface of the parasite. VSPs are cysteine-rich proteins containing many CXXC motifs, one or two GGCY motifs, a conserved hydrophobic tail and a Zn finger motif. The biological role(s) of VSPs is unclear. As VSPs are resistant to the effects of intestinal proteases, they likely allow the organism to survive in the protease-rich small intestine. Although immune escape is commonly mentioned as the reason antigenic variation occurs, VSP expression changes in vivo even in the absence of an adaptive immune system suggesting the biological role of antigenic variation is more complex. The molecular mechanisms involved in antigenic variation are not known but appear to differ from those known to occur in other protozoa.

Genetic manipulation of Giardia lamblia

Giardia lamblia is a flagellated protozoan that infects several species including humans and is a major agent of waterborne outbreaks of diarrhea. G. lamblia is also important in the study of basic eukaryotic molecular biology and evolution; however, it has been difficult to employ standard genetic methods in the study of Giardia. Over the past 6 years, two transfection systems were developed and used for the genetic manipulation of G. lamblia. Both systems allow transient or stable transfection of Giardia and/or foreign genes. The DNA-based transfection system allows electroporation of circular or linear plasmid DNA into trophozoites. The RNA virus-based transfection system requires electroporation of in vitro transcribed RNA into GLV-infected trophozoites. Because G. lamblia is one of the most rudimentary eukaryotes, its processes of transcription, translation and protein transport, as well as its metabolic and biochemical pathways, are of interest. Study of these areas will continue to be advanced using transfection in combination with cellular and molecular tools. Several groups have combined these technologies with other techniques to study protein transport and the transcriptional and post-transcriptional regulation of Giardia genes, including encystation-specific and variant surface protein genes. In addition, coupling antisense techniques with transfection has permitted functional knockout of Giardia metabolic genes, allowing Giardia metabolic pathways to be studied. In the near future, both transfection systems will be potent tools in our investigations of the perplexing questions in Giardia biology.

Characterization of a bi-directional promoter for divergent transcription of a PHD-zinc finger protein gene and a ran gene in the protozoan pathogen Giardia lamblia

We showed previously that transcription of the ran gene in Giardia lamblia is regulated by an AT-rich initiator. In the present study, the ran initiator was found to regulate transcription of a neighbouring PHD zinc-finger protein gene. Deletion and scanning mutagenesis of the phd promoter in a firefly luciferase reporter system showed that the promoter activity is determined by multiple single-stranded T-tract DNA elements distributed into a distal domain spanning the ran initiator (-134/-103) and a proximal domain (-88/-48) spanning phd messenger RNA (mRNA) start sites (-74, -55 and -53 relative to the first ATG). The promoter activity is repressed by the single T-tract element on a non-template strand of the ran initiator, and is activated by closely spaced T-tract elements on the opposite strand. The T-tract elements in the phd and ran initiators compete for similar ssDNA binding proteins. Mutation of -47/-42 resulted in dramatic reduction of luciferase activity without changing luciferase mRNA levels, indicating the potential involvement of a regulatory mechanism in PHD protein translation. These findings suggest that G. lamblia uses multiple copies of a T-tract element as both core and distal elements in regulating transcription initiation, and that expression of the phd gene is regulated at multiple levels.

Biology of Giardia lamblia

Giardia lamblia is a common cause of diarrhea in humans and other mammals throughout the world. It can be distinguished from other Giardia species by light or electron microscopy. The two major genotypes of G. lamblia that infect humans are so different genetically and biologically that they may warrant separate species or subspecies designations. Trophozoites have nuclei and a well-developed cytoskeleton but lack mitochondria, peroxisomes, and the components of oxidative phosphorylation. They have an endomembrane system with at least some characteristics of the Golgi complex and encoplasmic reticulum, which becomes more extensive in encysting organisms. The primitive nature of the organelles and metabolism, as well as small-subunit rRNA phylogeny, has led to the proposal that Giardia spp. are among the most primitive eukaryotes. G. lamblia probably has a ploidy of 4 and a genome size of approximately 10 to 12 Mb divided among five chromosomes. Most genes have short 5' and 3' untranslated regions and promoter regions that are near the initiation codon. Trophozoites exhibit antigenic variation of an extensive repertoire of cysteine-rich variant-specific surface proteins. Expression is allele specific, and changes in expression from one vsp gene to another have not been associated with sequence alterations or gene rearrangements. The Giardia genome project promises to greatly increase our understanding of this interesting and enigmatic organism.

Initiator and upstream elements in the alpha2-tubulin promoter of Giardia lamblia

Giardia lamblia, one of the earliest diverging eukaryotes and a major cause of diarrhea world-wide, has unusually short intergenic regions, raising questions concerning its regulation of gene expression. We have approached this issue through examination of the alpha2-tubulin promoter and in particular investigated the function of an AT-rich element surrounding the transcription start site. Its placement and the ability of this sequence to direct transcription initiation in the absence of any other promoter elements is similar to the initiator element in higher eukaryotes. However, the sequence diversity of extremely short (8-10 bp) initiator elements is surprising, as is their ability to independently direct substantial levels of transcription. We also identified a large AT-rich element located between -64 and -29 bp upstream of the transcriptional start site and show using both deletions and site-specific mutations of this region that sequences between -60 and the start of transcription are important for promoter strength; interestingly this AT-rich sequence is not highly conserved among different Giardia promoters. These data suggest that while the overall structure of the core promoter has been conserved throughout eukaryotic evolution, significant variation and flexibility is allowed in element consensus sequences and roles in transcription. In particular, the short and diverse sequences that function in transcription initiation in Giardia suggest the potential for relaxed transcriptional regulation.

Stage-specific expression and targeting of cyst wall protein-green fluorescent protein chimeras in Giardia

In preparation for being shed into the environment as infectious cysts, trophozoites of Giardia spp. synthesize and deposit large amounts of extracellular matrix into a resistant extracellular cyst wall. Functional aspects of this developmentally regulated process were investigated by expressing a series of chimeric cyst wall protein 1 (CWP1)-green fluorescent protein (GFP) reporter proteins. It was demonstrated that a short 110 bp 5' flanking region of the CWP1 gene harbors all necessary cis-DNA elements for strictly encystation-specific expression of a reporter during in vitro encystation, whereas sequences in the 3' flanking region are involved in modulation of steady-state levels of its mRNA during encystation. Encysting Giardia expressing CWP1-GFP chimeras showed formation and maturation of labeled dense granule-like vesicles and subsequent incorporation of GFP-tagged protein into the cyst wall, dependent on which domains of CWP1 were included. The N-terminal domain of CWP1 was required for targeting GFP to regulated compartments of the secretory apparatus, whereas a central domain containing leucine-rich repeats mediated association of the chimera with the extracellular cyst wall. We show that analysis of protein transport using GFP-tagged molecules is feasible in an anaerobic organism and provides a useful tool for investigating the organization of primitive eukaryotic vesicular transport.

Molecular basis of metronidazole resistance in pathogenic bacteria and protozoa

The molecular basis of metronidazole resistance has been examined in anaerobic bacteria, such as Bacteroides, Clostridium, and Helicobacter, and anaerobic parasitic protists such as Giardia, Entamoeba, and trichomonads. A variety of enzymatic and cellular alterations have been shown to correlate with metronidazole susceptibility in these pathogens; however, a common theme has been revealed. Resistant cells are typically deficient in drug activation. The frequent correlation between metronidazole resistance and ineffective drug activation suggests that drug resistance is the result of modification of proteins involved in drug activation. Copyright 1999 Harcourt Publishers Ltd.

Identification and characterization of a ran gene promoter in the protozoan pathogen Giardia lamblia

The promoter elements that regulate transcription initiation in Giardia lamblia are poorly understood. In this report, the promoter of the Giardia ran gene was studied using a luciferase expression plasmid pRANluc+ to monitor transcription efficiency. An AT-rich sequence spanning -51/-20 relative to the translation start site of the ran gene was identified and was found to be required for efficient luciferase expression by deletion and mutation mapping of pRANluc+. The -51/-20 sequence was also sufficient for promoter activity as revealed from studies on a 32-base pair synthetic promoter derived from this region. Deletion mapping of the synthetic promoter revealed two minimal promoter elements, -51/-42 and -30/-20, sufficient for 6- and 30-fold luciferase expression above background, respectively. The transcription start sites on luc+ messenger RNA were determined by the position of the synthetic promoter in the luciferase expression plasmids as shown by primer extension experiments. Results from electrophoretic mobility shift assays revealed multiple DNA-protein complexes upon binding of nuclear proteins with either DNA strand but not the double-stranded DNA derived from the ran promoter. Our results delineate the first promoter sequence of the Giardia gene (ran), which provides an excellent model for future studies on transcription regulation in this protozoan parasite.